EEG and the brain's state

EEG (Electroencephalography) technology is used to measure brain's electrical
vibrations from the surface of the scalp. The resulting EEG pattern will contain
frequency elements mainly below 30Hz. The frequencies are categorized into four
states as follows:

State

Frequency range

Amplitude

State of mind

Delta

0.5Hz - 4Hz

high (up to 200uV)

Deep sleep

Theta

4Hz - 8Hz

low (5uV - 20uV)

Drowsiness (also first stage of sleep)

Alpha

8Hz - 14Hz

high (up to 200uV)

Relaxed but alert

Beta

14Hz - 30Hz

low (less than 10uV)

Highly alert and focused

The dominant frequency in the EEG pattern determines what is called the current
state of the brain. If the amplitude of the alpha range frequencies is highest,
the brain is said to be in the alpha stage. Note that other frequencies still
occur; it is not meaningful to give any exact frequency your brain is "operating
on."